As shown in FIG. 1 (for example, Japanese Patent Application No. Sho 56-39807 (Patent Publication 1)), there is a drill for uniform material such as steel material in which two cutting chip discharging grooves 21 and 22 are formed spirally on an outer circumferential surface; land portions 23 and 24 are provided between the cutting chip discharging grooves 21 and 22; and furthermore, cut blades 29 and 30 are provided along the cross ridge portions between rake surfaces 25 and 26 directed in the drill rotational direction of the cutting tool discharging grooves 21 and 22 and relief surfaces 27 and 28 on the back side of the rake surfaces 25 and 26.
The two cutting chip discharge grooves 21 and 22 are normally provided symmetrically with respect to a point of a drill rotational center. Accordingly, with the type of the drill shown in FIG. 1, it is possible to perform the cutting in a good balance by using the point-symmetric blades and furthermore, it is possible to enhance the cutting chip discharge property to perform the boring work with high precision.
By the way, recently, a small drill for forming holes or the like to a printed circuit board has been spread widely. It is desired to thin and down-size the printed circuit board more and more. Thus, the small diameter drill itself is required to be smaller and smaller in size.
[Patent Publication] Patent Application Sho 56-39807
However, if the diameter of the drill is reduced, the thickness of the drill is also reduced so that the rigidity is deteriorated. It is therefore inevitable to degrade the precision in hole position.
Furthermore, in general, the printed circuit board is formed by impregnating glass cloth, formed by weaving glass fibers, with resin forming a prepreg and by laminating copper foil on the prepreg. The printed circuit board is made of non-uniform composite material unlike the above-described steel material or the like and undulations of the glass cloth sensitively affects the hole position precision. As a result, if the boring of the printed circuit board is performed with the small diameter drill, the hole position precision further deteriorates.
Accordingly, it is difficult to obtain the satisfactory hole position precision by providing two cutting chip discharge grooves and two cut blades to the small diameter drill, particularly, the small diameter drill for the printed circuit board.
In this connection, a structure is proposed to make only one cutting chip discharge groove for keeping a thickness (rigidity) to some extent (in this case, only one cut blade is made). However, in this case, it is difficult to keep a suitable cutting chip discharge path so that the cutting charge discharging property deteriorates. As a result, the cutting resistance is increased, the straight advancing property of the drill is degraded.